Burilko, O. A., and A. M. Samoilenko. “On the Smoothness of the Green Function for the Problem of Bounded Invariant Manifolds”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 4, Apr. 1998, pp. 570–584, https://umj.imath.kiev.ua/index.php/umj/article/view/4926.